For example, the gear of an automatic transmission or sheave of a continuously variable transmission, constant velocity joint, hub, or other power transmission part or other steel part is required to have a high bearing fatigue strength. In general, as the material for the above parts, JIS SCr420, SCM420, or other C: 0.2% or so case hardened steel is used. The case hardened steel is used treated by carburized quenching to form a C: 0.8% or so martensite structure hardened layer at the surface layer of the part so as to raise the bearing fatigue strength.
However, carburized quenching treatment is treatment at a 950° C. or so high temperature austenite region for 5 to 10 hours, in some cases 10 hours or more, so the crystal grains sometimes coarsen causing heat treatment deformation (quenching strain) to increase. For this reason, when a high precision is required, after the carburized quenching, the part has to be ground, honed, or otherwise finished.
In recent years, demand for making automobile engines etc. lower in noise has been rising, so surface hardening treatment such as induction hardening or soft nitridation with smaller thermal strain compared with carburized quenching treatment has come under the spotlight.
Induction hardening quenches only the required part of a surface layer part by a short period of heating to convert it to austenite, so the quenching strain is small. According to induction hardening, it is possible to obtain a surface hardened part with good precision. However, if using only induction hardening to obtain a strength equal to that of a carburized quenched material, a steel material which has a content of C of over 0.8% would be required. As a result, the base material would rise in hardness and the machineability would be remarkably degraded. Therefore, it is not possible to increase the content of C in the steel willy-nilly. There is a limit to improving the bearing fatigue strength by just induction hardening.
Soft nitridation treatment is treatment which obtains a surface hardened layer at a temperature range of the A1 transformation point or less. Further, compared with carburized quenching treatment, the treatment time is a short 2 to 4 hours or so. For this reason, soft nitridation is often applied to steel parts from which low strain is demanded. However, the hardened layer depth which is obtained by soft nitridation treatment alone is small, so it is difficult to apply this to transmission gears etc. where a high bearing pressure is applied.
Recently, as a technique which makes up for the defects of induction hardening and soft nitridation treatment and obtained better mechanical properties, in particular bearing fatigue strength, it has been attempted to perform soft nitridation followed by induction hardening.
PLTs 1 to 3 disclose steel for machine structural use where soft nitridation treatment and induction hardening are combined to improve the bearing fatigue strength. In the art which is described in PLTs 1 to 3, the induction hardening temperature is less than 950° C., so the nitrides which precipitated at the surface layer due to the soft nitridation treatment did not sufficiently dissolve. Most of the nitrogen (N) remained present as nitrides. The dissolved N concentration of the surface layer was low. As a result, the compressive residual stress was low, so a sufficiently high bearing fatigue strength could not be obtained.
PLT 4 proposes a process of production of a steel part which is excellent in mechanical strength by a combination of induction hardening and nitridation treatment. The steel part which is obtained by the process of production of PLT 4 had a high surface layer hardness. However, the total N concentration at the surface layer, that is, the total of the N concentration of nitrides and the dissolved N concentration, was low, and V and other nitride-forming elements were present in large amounts. Further, the dissolved N concentration in the total N concentration of the surface layer was low, so the high temperature hardness of the surface layer was low. For this reason, it was not possible to obtain a sufficient temper softening resistance at the surface layer of a gear etc. becoming high in temperature during operation and not possible to obtain a high bearing fatigue strength.
PLT 5 also proposes art which combines induction hardening and nitridation treatment so as to obtain excellent mechanical properties. The art which is described in PLT 5 is characterized by a high concentration of nitrogen from the surface to 0.05 mm depth. However, bearing fatigue fracture is fracture starting from the surface, but the depth of fracture reaches several times the depth of 0.05 mm. Therefore, with just a high hardness from the surface to a depth of 0.05 mm, a high bearing fatigue strength cannot be obtained.
PLT 6 also proposes a process of production of a steel part which is excellent in mechanical strength by a combination of induction hardening and nitridation treatment. The art which is described in PLT 6 gives a deeper effective hardened layer for the size of the part and a deeper depth occupied in the region of austenization by high frequency heating. For this reason, the compressive residual stress in the vicinity of the surface is small and furthermore the quenching strain becomes large. This is not preferable as part characteristics.